Technologies for determining deterioration of an oxidation catalyst, which is based on the temperature of the oxidation catalyst observed when fuel is added to the oxidation catalyst disposed at an exhaust passage of an internal combustion engine, are known. (for example, see Patent Document 1.) However, the addition of fuel may cause release of the fuel into the atmosphere and lower fuel economy.
When determination of deterioration of an NOx selective reduction catalyst is carried out in cases where the NOx selective reduction catalyst is disposed at a location downstream of an oxidation catalyst, a more accurate determination is possible by taking the degree of deterioration of the oxidation catalyst into account. For example, in cases where determination of deterioration of the NOx selective reduction catalyst is carried out based on the NOx removal rate of the NOx selective reduction catalyst, the NOx removal rate varies depending on the ratio between the NO and NO2 which inflow into the NOx selective reduction catalyst. Since this ratio between NO and NO2 varies depending on the degree of deterioration of the oxidation catalyst, a decrease in the NOx removal rate of the NOx selective reduction catalyst might be due to deterioration of the oxidation catalyst. That is, it is important to determine which catalyst is deteriorated.    Patent Document 1: JP 2001-263048 A    Patent Document 2: JP 2005-23921 A    Patent Document 3: JP 2004-100700 A    Patent Document 4: Japanese Translated PCT Patent Application Laid-open No. 2008-523305